Friday, October 02, 2009

The fossil, known as 'Ardi', is the oldest specimen on the hominid branch that led
to modern humans yet unearthed

by Hannah Devlin

She had a stocky, ape-like form and a protruding jaw, with huge hands and a stooping gait. Beautiful she was not.

But Ardi, as scientists have christened her, has emerged as one of our oldest and most important ancestors.

The question of what our first “hominid” forebears looked like has intrigued scientists since the birth of evolutionary theory 150 years ago. But it is only now, with the discovery of a 4.4 million-year-old skeleton, that scientists have been able to paint a vivid picture of them.

The Ardipithecus ramidus fossil is the oldest specimen on the hominid branch found to date, and the closest that we have got to tracing our roots back to “the missing link” — our last common ancestor with modern apes.

The fossil is more than a million years older than the so-called “Lucy” skeleton. The lines that evolved into modern humans and apes last shared an ancestor at least six to seven million years ago.

Tim White, of the University of California Berkeley and a lead author of the study, said: “Darwin was very wise on this matter. He said, ‘We have to be really careful. The only way we are really going to know what this last common ancestor looked like is to go and find it’. Well, we have not found it. But we have come closer than we have ever come.”

The adult female specimen was found in 1994 in Afar, in the Great Rift Valley of northeastern Ethiopia.

It took an international team of researchers nearly three years to trawl the volcanic ash where the fossil lay and piece together the 125 fragments. The significance of the find meant that scientists took a further 13 years analysing the specimen, which is described today in intricate detail in the journal Science.

Ardi would have been about 4ft tall, and weighed about 9st, almost a foot taller and twice the weight of Lucy. Her brain was only slightly larger than a modern chimp’s and considerably smaller than Lucy’s, suggesting that our ancestors evolved an advanced intellect much later.

The angle of her head relative to her spine and the position of her pelvis and hip show that Ardi would have been able to walk with a stooped posture.

However, she retains the “grasping” big toe of our more primitive ancestors, as well as long arms and big hands, which suggest that she was an able climber, and would have spent much of her time in the trees. Unlike chimpanzees and orang-utans, though, she would not have been able to swing through the branches.

Analysis of Ardi’s teeth suggests that she lived on a diet of fruit, leaves and small mammals. Her canine teeth, and isolated male canine teeth found at the site, were similar in size and very small. This reveals an intriguing departure from modern apes, such as chimpanzees and gorillas, where males possess large canines that they bare in conflicts over females.

The comparable size of canine teeth between males and females suggests a low level of male aggression in hominid societies of the time. Males and females were likely to have had co-operative, monogamous relationships in which the males were involved in raising the young.

“This fossil suggests that the ‘feminisation’ of the canines, which is unique to humans, goes back deep into our history,” said Chris Stringer, head of human origins at the Natural History Museum in London.

The fossil also changes thinking about the habitat of early hominids. It was previously thought that our ancestors developed upright walking as a consequence of living in the grassland savannahs. However, Afar would have been a lush forest environment populated by monkeys and elephants. Source